ABSTRACT Regulation of keratinocyte proliferation and differentiation is important to maintain skin function. Retinoic acids, the active form of retinoids, play a role in this regulation by inducing or repressing the expression of hundreds of genes. Retinoic acid activity is mediated by binding to retinoic acid receptors that bind to regions within gene promoters. Because of their activity, retinoids have been used in the treatment of a variety of skin disorders including acne, psoriasis, ichthyoses, keratodermas, and skin cancers, and are utilized in treatments to reduce photoaging effects. Clinically prescribed retinoids are developed to modulate the metabolism of endogenous retinoids or to directly activate retinoid receptors. Because of this, it is important to understand both the activity and metabolism of endogenous retinoids. Currently, much of this information is not known for 3,4- dehydroretinoids, which make up approximately 25% of the total epidermal retinoid pool. 3,4-Dehydroretinol is a metabolite of all-trans retinol produced in keratinocytes, and our laboratory has only recently identified cytochrome P450 (CYP) 27C1 as the desaturase responsible for this reaction. 3,4-Dehydroretinol can be further metabolized to form an active 3,4-dehydroretinoic acid that binds to retinoic acid receptors, but few studies have investigated dehydroretinoid-specific gene regulation. Additionally, increased concentrations of 3,4- dehydroretinol in the skin have been associated with skin diseases including psoriasis, squamous cell carcinoma, and keratoacanthoma. Overall, the goal of this study is to provide a better understanding of the regulation and function of dehydroretinoids through characterizing CYP27C1. Our lab recently determined that human CYP27C1 is preferentially expressed in the skin and catalyzes the desaturation of all-trans retinoids to form 3,4- dehydroretinoids in vitro in the presence of mitochondrial P450 redox partners. Many aspects of the CYP27C1 catalytic mechanism are well understood, but questions remain regarding the biological function of this enzyme. I hypothesize that mitochondrial CYP27C1 regulates keratinocyte differentiation and proliferation through catalyzing the formation of 3,4-dehydroretinoids. A combination of biochemical, analytical, and molecular biology tools including primary cell culture, proteomics, RNA-sequencing, and immunoassays will be used to address my hypothesis. I propose three aims: 1) localization and absolute quantification of CYP27C1 within the skin, 2) investigate UV light and retinoids as effectors of CYP27C1 expression, and 3) evaluation of the physiological function of CYP27C1 and dehydroretinoids in the skin. Studying CYP27C1 in primary human epidermal keratinocytes in this proposed work will add to the knowledge-base of physiological retinoid homeostasis and may aid in the development of new therapies for dermatological diseases.